- add more documentation to MutableDict explaining that this structure is only intended to track additions and removals from the dictionary, not recursive tracking of embedded changes.  fixes #3646.
- add more documentation to MutableDict explaining
that this structure is only intended to track additions
and removals from the dictionary, not recursive tracking
of embedded changes.  fixes #3646.

zzzeek/sqlalchemy,bdupharm/sqlalchemy,sqlalchemy/sqlalchemy,wujuguang/sqlalchemy,j5int/sqlalchemy,monetate/sqlalchemy,robin900/sqlalchemy,j5int/sqlalchemy,graingert/sqlalchemy,graingert/sqlalchemy,sandan/sqlalchemy,sandan/sqlalchemy,robin900/sqlalchemy,bdupharm/sqlalchemy,monetate/sqlalchemy,wujuguang/sqlalchemy